According to the present-day technique for extracting the uranium contained in uranium-bearing ores, the uranium, after being purified and concentrated, is recovered in the form of acid or alkaline liquors. The uranium-bearing concentrates are precipitated from these acid or basic solutions in a medium of SO.sub.4 --and/or CO.sub.3 --by means of sodium hydroxide, in particular. The precipitation is carried out in a series of tanks arranged in a cascade, maintained at 60.degree.-80.degree. C. This results in a very fine precipitate of sodium uranate which is difficult to filter and dry, and which has a particle size of between 0 and 15 .mu.m. These particles are irregular in shape and the product obtained has poor flow properties. Handling such a product involves hygiene and safety problems because of the emission of dust which is rich in uranium, both in the factory where the uranium-bearing concentrate is produced, when the product is dried and the solid is placed in a cask, and also in the refining plants which take these uranium-bearing concentrates in order to purify them and convert them into the oxide, fluoride or metal.
Various methods are known for bringing the crystallized substances into the physical state required by the user. The technique of agglomerating fine particles in a fluidized bed is one of the most effective. It consists in forming polycrystalline agglomerates from fine particles injected into a solution of the product to be crystallized with a suitable supersaturation. These fine particles grow and become welded together to form polycrystalline agglomerates of a specific particle size. These fine particles and agglomerates are kept in suspension by means of a given flow rate of supersaturated solution which is fed in or recycled in. The state of supersaturation remains constant thanks to the continuous addition of the substance to be crystallized. This technique is described in French Pat. No. 1,187,352, of 29.11.1957, by Produits Chimiques Ugine Kuhlmann.
If this technique of crystallization in a fluidized bed is applied to uranium-bearing solutions, it does not lead to the formation of polycrystalline agglomerates, but considerable nucleation of very fine particles occurs. Great dilution of the uranium-containing solutions is one method of reducing the frequency of nucleation so as to obtain particles of sufficient diameter. However, this solution is difficult to apply on an industrial scale as there is a conflict between the conditions required for good fluidization and the conditions which correspond to adequate productivity per unit of volume of the apparatus. Moreover, dilution cannot be carried out using water, which would weigh down the water balance of the process, and conventional dilution with mother liquors involves a risk of substantial nucleation, as there is always an excess of sodium hydroxide in the recyclable mother liquors.